As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high voltage, extended service-life, and low self discharge rate, which has been commercialized and widely used.
Based on the construction of electrodes and an electrolyte, the lithium secondary battery may be classified as a lithium-ion battery, a lithium-ion polymer battery, or a lithium polymer battery. Among them, the lithium-ion polymer battery has been increasingly used because the lithium-ion polymer battery has a low possibility of electrolyte leakage and can be easily manufactured.
An electrode assembly having a cathode/a separator/an anode structure, which constitutes the secondary battery, may be generally classified as a jelly-roll (winding) type electrode assembly or a stacking type electrode assembly based on the structure of the electrode assembly. The jelly-roll type electrode assembly is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and winding the anode/separator/cathode structure. The jelly-roll type electrode assembly is suitable for cylindrical cells; however, the jelly-roll type electrode assembly is not suitable for prismatic cells or pouch-shaped cells because the electrode active material may be detached, and the spatial utilizability is low. On the other hand, the stacking type electrode assembly is an electrode assembly constructed in a structure in which a plurality of cathode and anode units are sequentially stacked one on another. The stacking type electrode assembly has an advantage in that the stacking type electrode assembly can be constructed in a prismatic structure; however, the stacking type electrode assembly has disadvantages in that a process for manufacturing the stacking type electrode assembly is complicated and troublesome, and, when external impacts are applied to the stacking type electrode assembly, electrodes of the stacking type electrode assembly are pushed with the result that short circuits occur in the stacking type electrode assembly.
In order to solve the above-described problems, there has been developed an electrode assembly having a novel structure, which is a combination of the jelly-roll type electrode assembly and the stacking type electrode assembly, i.e., an electrode assembly constructed in a structure in which full cells having a cathode/separator/anode structure of a predetermined unit size or bicells having a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure are folded using a long continuous separation film. Examples of such an electrode assembly are disclosed in Korean Unexamined Patent Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application. Hereinafter, the electrode assembly having the above-described structure will be referred to as a hybrid type electrode assembly.
A secondary battery having the above-described stacking type or hybrid type electrode assembly mounted in a battery case may constructed in various shapes. A representative example of the secondary battery may be a lithium-ion polymer battery (LiPB) having a pouch-shaped case made of an aluminum laminate sheet.
The lithium-ion polymer battery is constructed in a structure in which an electrode assembly manufactured by thermally welding electrodes (cathodes and anodes) and separators is impregnated with an electrolyte. Mostly, the lithium-ion polymer battery is constructed in a structure in which the stacking type or the hybrid type electrode assembly is mounted in the pouch-shaped case made of the aluminum laminate sheet in a sealed state. For this reason, the lithium-ion polymer battery is often referred to as a pouch-shaped battery.
FIGS. 1 and 2 typically illustrate a general structure of a representative lithium-ion polymer battery including a stacking type electrode assembly.
Referring to these drawings, a lithium-ion polymer battery 100 is constructed in a structure in which an electrode assembly 300 including cathodes, anodes, and separators disposed between the cathodes and the anodes is mounted in a pouch-shaped battery case 200, cathode and anode taps 310 and 320 of the electrode assembly 300 are welded to two electrode leads 400 and 410, respectively, and the electrode assembly 300 is sealed in the battery case 200 while electrode leads 400 and 410 are exposed to the outside of the battery case 200.
The battery case 200 is made of a soft wrapping material, such as an aluminum laminate sheet. The battery case 200 includes a case body 210 having a hollow receiving part 230 for receiving the electrode assembly 300 and a cover 220 connected to the case body 210 at one side thereof.
The electrode assembly 300 of the lithium-ion polymer battery 100 may be constructed in the previously-described jelly-roll type structure in addition to the stacking type structure shown in FIG. 1. The stacking type electrode assembly 300 is constructed in a structure in which the cathode taps 310 and the anode taps 320 are welded to the electrode leads 400 and 410, respectively, and insulative films 500 are attached to the upper and lower surfaces of the electrode leads 400 and 410 for securing electrical insulation and sealing between the electrode leads 400 and 410 and the battery case 200.
When a lithium secondary battery, such as the lithium-ion polymer battery, is exposed to high temperature, or when a large amount of current flows in a short time due to overdischarge, an external short circuit, a nail penetration, a local crush, or a drop-induced short circuit, the battery is heated due to IR heat generation with the result that the battery may catch fire or explode. As the temperature of the battery is increased, the reaction between the electrolyte and the electrodes is accelerated. As a result, heat of reaction is generated, and therefore, the temperature of the battery is further increased, which accelerates the reaction between the electrolyte and the electrodes. As a result, the temperature of the battery is sharply increased, and therefore, the reaction between the electrolyte and the electrodes is accelerated. This vicious cycle causes a thermal runaway phenomenon in which the temperature of the battery is sharply increased. When the temperature of the battery is increased to a predetermined temperature level, the battery may catch fire. Also, as a result of the reaction between the electrolyte and the electrodes, gas is generated, and therefore, the internal pressure of the battery is increased. When the internal pressure of the battery is increased to a predetermined pressure level, the lithium secondary battery may explode. This possibility that the lithium secondary battery catches fire and explodes is the most fatal disadvantage of the lithium secondary battery.
Especially, the battery case of the lithium-ion polymer battery is made of a soft wrapping material having low strength. As a result, the battery case of the lithium-ion polymer battery is easily deformed when the battery case falls or external impacts are applied to the battery case. As shown in FIG. 2, a space 230a is provided at the upper end of the electrode assembly 300 in the battery case 200 such that the electrode taps of the electrode assembly are connected to the electrode leads 400 and 410 by welding in the space 230a. Consequently, when external impacts are applied to the battery at the upper end of the battery due to the falling of the battery, the electrode assembly 300 is moved toward the upper end space 230a, and, therefore, the electrode leads 400 and 410 are brought into contact with the upper end or the outermost electrodes of the electrode assembly 300 with the result that an internal short circuit may occur. The falling of the battery frequently occurs during the use of the battery. Consequently, the demand of a technology for more efficiently securing the safety of the battery is very high.
Some of conventional arts propose a method of attaching adhesive tape to predetermined positions of the electrode assembly and a method of filling the upper space of the electrode assembly with a foreign material, in order to prevent the internal short circuit due to the movement of the electrode assembly. However, these methods have a problem in that the adhesive tape and the foreign material chemically react with the electrolyte, and therefore, the performance of the battery is reduced.
In addition to the above safety-related problem, one of the problems caused during the manufacture of the battery is the impregnation of the electrodes with the electrolyte, i.e., the wetting characteristic.
For example, the lithium secondary battery uses metal oxide, such as LiCoO2, as a cathode active material, and carbon as an anode active material. Polyolefin-based porous separators are disposed between the anodes and the cathodes, and a non-aqueous electrolyte including lithium salt, such as LiPF6, is injected into the lithium secondary battery. In this way, the lithium secondary battery is manufactured. During the charge of the lithium secondary battery, lithium ions are discharged from the cathode active material and inserted into a carbon layer of the anode. During the discharge of the lithium secondary battery, on the other hand, lithium ions are discharged from the carbon layer of the anode and inserted into the cathode active material. At this time, the non-aqueous electrolyte serves as a medium to move the lithium ions between the anode and the cathode. It is necessary for the lithium secondary battery to be basically stable within the operating voltage range of the battery and have a performance to transfer ions at sufficiently high speed.
The non-aqueous electrolyte is injected into the battery at the final stage of manufacturing the lithium secondary battery. At this time, the electrodes must be rapidly and completely wetted by the electrolyte so as to reduce the time necessary for manufacturing the battery and optimize the performance of the battery.
A non-protic organic solvent, such as ethylene carbonate, diethyl carbonate, or 2-methyl tetrahydrofuran, is mainly used as the non-aqueous electrolyte of the lithium secondary battery. Such an electrolyte is a polar solvent having polarity enough to effectively dissolve and dissociate the electrolyte salt, and, at the same time, a non-protic solvent having no active hydrogen. This electrolyte has high viscosity and surface tension due to wide interaction in the electrolyte. Consequently, the non-aqueous electrolyte of the lithium secondary battery has a low affinity for an electrode material including a polytetrafluoroethylene and polyvinylidene fluoride bonding agent, and, as a result, the electrode material is not easily wetted by the non-aqueous electrolyte. This is one of principal factors to ineffectively increase the time necessary for manufacturing the battery.
When the activating operation of the battery is carried out while the electrodes are not efficiently wetted by the electrolyte, a solid electrolyte interface (SEI) film is not properly formed at the anode, and therefore, the service life of the battery is reduced.
For this reason, an additional process, such as aging, for maintaining the wet state of the anode, such that the anode can be sufficiently wetted by the electrolyte, after the injecting the electrolyte, or a special process for applying vacuum or pressure to the electrodes has been performed in order to accelerate the wetting of the electrolyte to the electrodes.
The hybrid type electrode assembly constructed as described above has many advantages; however, the hybrid type electrode assembly also has a disadvantage in that the separation film covers the sides of the electrodes, and therefore, the electrolyte can access only to the upper and lower ends of the electrodes. Generally, the sides of the electrodes are longer than the upper and lower ends of the electrodes. In the hybrid type electrode assembly, therefore, the area of the electrode contacting the electrolyte is reduced with the result that the wetting operation of the electrolyte is inevitably lengthened.